JAR-FCL 1.055
CHIEF GROUND INSTRUCTOR (CGI)
19 The CGI shall be responsible for the supervision of all ground instructors and for the standardisation of all theoretical knowledge instruction. The CGI shall have a practical background in aviation and have undergone a course of training in instructional techniques or have had extensive previous experience in giving theoretical knowledge instruction.
THEORETICAL KNOWLEDGE INSTRUCTORS
20 Theoretical Knowledge Instructors in licence and ratings examination subjects shall have appropriate experience in aviation and shall, before appointment, give proof of their competency by giving a test lecture based on material they have developed for the subjects they are to teach.
You have to convince the CGI to employ you, and the CGI has to convince the CAA that you have adequate experience and competence to be acceptable. Don't forget you also need credibility with your students.